5th generation, or 5G, cellular service will require several GHz of bandwidth to be supplied to individual cellular antennas. Radio over fiber (RoF) technology has the ability to scale to such high bandwidth requirements due to the THz of bandwidth provided by fiber optic cables. The fiber channel can also provide wavelength division multiplexing to accommodate a high number of wireless channels. Transmitting analog signals directly to a cellular antenna for transmission into free-space instead of transmitting digital data to the antenna and then converting the digital data to analog at the top of the antenna has an advantage of removing complexity from the remote antenna. In such RoF transmission systems, the digital to analog conversion may be done at a central office or at a remote distribution unit and an optical detector at top transmitter's antenna converts the transmitted RF optical signal to an RF electrical signal for use in driving the antenna.
Several techniques have been proposed for the digital to analog conversion/modulation for use in RoF transmission systems. Some silicon photonics based modulators use the radio frequency (RF) digital electrical signals to directly control multiple phase shifters in an arm of a Mach-Zehnder interferometer (MZI) structure, which results in modulating an amplitude of the output optical signal. However, incorporating the electrically controlled phase shifters into a single MZI structure requires the individual phase shifters to be disposed in a close physical proximity to each other, which may result in crosstalk between the RF signals applied to individual phase shifters. Other modulators may reduce the RF crosstalk by using separate optical wavelengths for each digital bit stream, which allows the RF electrical bit signals to be physically separated; however such modulation results in spectral inefficiency since each individual bit stream is modulated by a separate wavelength and as such a multi-bit signal will be modulated on multiple different wavelengths. Further, walk-off between different wavelengths over a long length of fiber would require compensation, increasing the system complexity.
An additional, alternative and/or improved digital to analog modulator for use in converting a multi-bit digital electrical signal to a corresponding analog optical signal is desired.